1. Field of the Invention
The present invention relates generally to an ink-jet printing apparatus. More particularly, the invention relates to an ink-jet printing apparatus which has an air buffering vessel in an ink supply system supplying an ink to a printing head.
2. Description of the Related Art
Nowadays, various printing systems have been developed as printing apparatus. Amongst, an ink-jet printing system has been widely employed for superiority in gentleness, capability of high speed and high density printing for non-contacting with a printing medium, and easiness of colorization and down-sizing.
One system of the conventional ink-jet printing apparatus is constructed to reciprocate a printing head mounted on a carriage and perform printing on the printing medium by ejecting an ink from the printing head. In such conventional ink-jet printing apparatus, it is necessary to increase capacity of an ink tank in order to reduce frequency of exchanging of the ink tank. However, an integral type of the printing head and the ink tank, when the capacity of the ink tank is increased, inertial mass on the carriage is inherently increased. Therefore, in order to avoid increasing of inertial mass, the ink tank is arranged at stationary or fixed position without requiring movement and supplies the ink to the printing head on the carriage through a tube or the like by means of various pumps.
On the other hand, in the ink-jet printing apparatus, for avoiding dripping of ink from the printing head except for driving for printing, the inside of the printing head is constantly maintained in a slight negative pressure state lower than an atmospheric pressure. By this, ejection upon driving for printing and prevention of unnecessary dripping other than driving for printing can be achieved.
However, when the carriage is moved reciprocally in the ink-jet printing apparatus for printing, the ink within the tube connecting the printing head and the ink tank flows to cause fluctuation of an ink ejection amount by pushing out and pulling back the ink within the printing head, resulting in fluctuation of printing or ejection failure.
Accordingly, as a measure for such fluctuation of printing or ejection failure, there has been known a technology, in which an air buffering vessel having an air storing portion is arranged between the tube and the printing head in the vicinity of the printing head for buffering pressure variation due to flow of the ink within the tube utilizing elastic property of the air to stabilize ejection.
Such an air buffering vessel defines an air storing portion in the upper portion and an ink storing portion in the lower portion thereof. The air storing portion also has a function for separating air from the ink admixing the air within the tube to accumulate within the air storing portion for feeding only ink to the printing head. Accordingly, so as to avoid penetration of the air into the printing head from the ink storing portion due to waving of a liquid level of the ink storing portion by reciprocal motion of the carriage, the height of the liquid level in the ink storing portion has to be maintained at a predetermined height.
On the other hand, even in an ink-jet printing apparatus of the type performing printing by feeding a printing medium without moving a carriage, namely a printing head, a similar vessel is arranged for separating the air admixed in the ink. The internal ink storing portion in the vessel is required to certainly maintain the height of the liquid level at the predetermined height for the purpose of air separation in either of the carriage scanning type and the stationary printing head type printing apparatus.
However, the height of the liquid level tends to be gradually lowered due to penetration of air from a tube wall surface or degassing of dissolved gas in the ink. Therefore, in order to continuously maintain the predetermined liquid level height, there is a known method, in which two tubes are used to connect one ends of respective tubes to the bottom surface position and the predetermined liquid level height position of the ink storing portion respectively and the other ends to a separate stationary ink tank to circulate the ink by the pump.
However, in the conventional method for maintaining the liquid level height using two tubes, number of tubes connecting the printing head mounted on the carriage and the stationary ink tank which is not movable, must be increased. Particularly, in the case of a color printer of a plurality of colors, problems that an arrangement of the tubes becomes complicate, a driving resistance of the carriage due to increasing of stiffness increases and the parts assembling cost elevates, can be encountered.
The foregoing problems will not be encountered when using a single tube having low air permeability. However, at present, an appropriate material having no air permeability in the tube having bending ability and flexibility, is hardly available. Accordingly, it is difficult to maintain the predetermined liquid level height over a long period of time without liquid level height adjusting function employing a single low cost tube.